The present invention relates to electron beam evaporation sources wherein an evaporant contained within a crucible is evaporated by an electron beam for deposition onto a substrate. Even more particularly, the present invention relates to an electron beam evaporation source in which the electron beam is vertically deflected into the crucible, through an arc of about 270 degrees, and is deflected in a horizontal plane for impact at predetermined points on the top surface of the evaporant in order to fully utilize the evaporant.
The prior art has provided electron beam evaporation sources that are used in depositing metallic and nonmetallic materials onto a substrate such as in the manufacture of integrated circuits, the coating of ophthalmic lenses, etc. A common design of an electron beam evaporation source utilizes a crucible for containing the material to be evaporated, such material being referred to in the art as an evaporant, and an electron beam emitted by an emitter of an electron beam gun that is vertically deflected through an arc of 270.degree. and into the crucible. The electron beam is vertically deflected by a transverse magnetic field produced by a pair of elongated pole pieces located on opposite sides of the crucible and transversely connected by a permanent magnet. The electron beam gun is located beneath a cover plate connecting the pole pieces in order to prevent the evaporant from spattering onto the emitter.
One important application for electron beam evaporation sources is in molecular beam epitaxy (MBE) in which a thin layer of evaporant is slowly and uniformly deposited onto a substrate in an ultra high vacuum environment, that is in a vacuum environment of 10.sup.-8 torr and below. MBE depositions are utilized for providing uniform doping layers at the junctions of semiconductors and also, in the manufacture of layered integrated circuits. Another important application of electron beam evaporation sources is in the manufacture of ophthalmic lenses. In such application, many thin and uniform layers of evaporant are deposited onto a lens in a high vacuum environment, that is in a vacuum environment of 10.sup.-7 torr and above. In both of such applications, a large crucible is utilized, typically having a volume of 150 cc and above. In the case of MBE the large crucible is utilized because it normally takes about two weeks to achieve an ultra high vacuum; and thus, it is not practical to stop production and to replenish the crucible before the evaporant has been fully utilized. In ophthalmic lens manufacturing technology the large crucible is utilized because of the amount of evaporant to be deposited.
In any electron beam evaporation source, the electron beam bores into the evaporant at the impact point of the electron beam on the evaporant. This is particularly the case when the evaporant comprises a subliming material such as silica. Thus, in order to fully utilize the evaporant in the crucible, particularly in a large crucible employed in the foregoing applications, the electron beam is deflected in a horizontal plane, namely in a plane at right angles to the pole pieces or more exactly, one extending across the crucible. Such deflection is accomplished in a predetermined manner in order to selectively reposition the impact point of the electron beam on the evaporant. To this end, the prior art has provided for the electromagnetic deflection of the electron beam through the use of a U-shaped arrangement of electromagnets located adjacent to the emitter of the electron beam gun. The electromagnets are differentially energized in order to deflect the electron beam in a horizontal plane.
The problem associated with deflecting the electron beam in a horizontal plane is that the impact area of the electron beam on the top surface of the evaporant distorts in dependence on the position of the impact area. Such distortion of the electron beam is produced by nonuniformities in the magnetic field vertically deflecting the electron beam through the arc of 270 degrees and in the magnetic field of the U-shaped electromagnets used in deflecting the electron beam in a horizontal plane. As a result of this problem, the power density of the electron beam at the impact area and thus, the rate of evaporation, changes in dependence upon the position of the impact area. This problem is particularly critical in MBE and ophthalmic coating in which uniformity in the thickness of the deposited layers is required. Thus, in the prior art, relatively complex monitoring systems are required to monitor the evaporation rate of the evaporant and to accordingly adjust the power density of the electron beam in each position of the electron beam so that the evaporation rate remains constant.
As will be discussed, the present invention solves the differential evaporation rate problem associated with the deflection of the electron beam in the horizontal plane by providing an electron beam evaporation source in which the size of the impact area of the electron beam remains essentially constant in any selected position of the impact area. Since the size of the impact area remains substantially uniform, the power density of the beam at the impact point of the electron beam on the evaporant remains essentially constant as does the evaporation rate of the evaporant.